def IsPrime(a):
    if a==1:return False
    test=True
    k=a-1
    while k>1:
        if not a%k:
            test=False
            break
        k-=1
    return test

n=input()
r=[]
for i in range(len(n)):
    for k in range(i+1,len(n)+1):
        a=int(n[i:k])
        if IsPrime(a):
            r+=[a]
print(max(r) if r else 0)